An idler is primarily consisted of a fixed housing, tension arms, a roller, torsion springs, rolling bearings and a flexible shaft sleeve and other components. The idler can automatically adjust the tensile force according to different tightness degrees of a belt, to ensure that a transmission system is stable and reliable. The idler is a rapid wearable part as auto accessories, and the belt is easy stretched after working for an extended period. Some of the idler can automatically adjust the tensile force of the belt, and with the idler, the belt can run more smoothly, its noise is reduced, and slipping may be prevented. As the role of the idler is used to adjust the tightness of a timing belt, it is usually replaced with the timing belt. While other parts only need regular maintenance if not replaced, the oil must be frequently changed, e.g. in general, the oil should be renewed while the driving distance is more than four thousand to five thousand kilometers. An conventional idler of this type commonly employs two half-round rollers riveted together. However, using such manufacturing method, will lead to the following shortcomings and deficiencies: an excessive number of rivets and low productivity; and the idler is deformed after riveting, the accuracy of a product becomes low, and a flat groove surface of the two half-round rollers is not smooth, resulting in the belt prone to be worn and the life of the belt to be reduced. Moreover, the surface that two half-round rollers contacting with bearing parts is small, so bearings are easy to be loosen or jammed after assembled, and the life of the bearings is short. In addition, using a method of stamping and then welding, the idler would have the residual stress and a deformation. By searching, the inventor has found no relevant patents but only one referenced application numbered with CN201610377854.0 to the present invention. The CN201610377854.0 is disclosed with a forming process of idler only by extrusion, the method will damage the toughness of metal material more or less, and cannot ensure the strength of the idler.